Synapse is a junction between two neurons or a neuron and a target or effector cell such as a muscle cell. It permits transmission of electrical or chemical signals.
The synapse is formed between presynaptic and postsynaptic neurons. It is known as the neuromuscular junction between a neuron and muscle.
The conduction of nerve impulses from an axon terminal of a neuron to dendrites of the next neuron occurs through a synapse. It can be electrical or chemical.
- Chemical synapses are more common. The transmission of nerve impulses through chemical synapses is mediated by neurotransmitters.
- There is a fluid-filled space between the two neurons called the synaptic cleft. The nerve impulse cannot jump from one neuron to another.
- Axon terminals have a knob-like structure, which contain synaptic vesicles.
- When the action potential reaches the terminals, the synaptic vesicles from the terminal of the presynaptic neuron release neurotransmitters at the synaptic cleft.
- Neurotransmitters bind to the receptors present in the postsynaptic membrane.
- This results in the opening of voltage-gated channels and the flow of ions. This causes a change in the polarisation of the postsynaptic membrane and the electric signal is conducted across the synapse.
- Neurotransmitters can be inhibitory or excitatory. One neurotransmitter can initiate different responses in different cells.
- If there is a net flow of positively charged ions inside the cell, then the neurotransmitter is excitatory and it results in generating the action potential. This is called EPSP or excitatory postsynaptic potential.
- When the membrane potential becomes more negative, the membrane becomes hyperpolarized and the action of neurotransmitter is inhibitory. They generate IPSP or inhibitory postsynaptic potential.
- Once the neurotransmitters get attached to the receptor they are either acted on by enzymes or taken back and recycled to terminate the signal after it is conducted forward.
- Electrical synapses are faster than chemical synapses.
- The presynaptic and postsynaptic neurons are in close proximity and they form gap junctions. There is a physical connection between pre and postsynaptic neurons at the gap junction by protein channels.
- These gap junctions allow the direct flow of ions and the transmission of an electric signal across the electrical synapse is similar to the conduction of impulse in an axon.
- Electrical synapses are not as flexible as chemical synapses as they cannot turn the excitatory signal to the inhibitory signal.
- It is found in some lower invertebrates. In humans, it is found between glial cells.
The synapse, in this way, ensures the conduction of nerve impulses in the correct direction and prevents random stimulation.
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